The present invention relates to a novel drug delivery system. More particularly, the present invention relates to a product in which a biologically active material is present in a multiphase system, i.e., (a) captured in multilamellar lipid vesicles (MLV); (b) dissolved in the solvent components of the system; and (c) in a solid crystalline or amorphous state.
Liposomes are lipid vesicles composed of membrane-like lipid layers surrounding aqueous compartments. Liposomes are widely used to encapsulate biologically active materials for a variety of purposes, but particularly they are used as drug carriers. Depending on the number of lipid layers, size, surface charge, lipid composition and methods of preparation various types of liposomes have been utilized.
Multilamellar lipid vesicles (MLV) were first described by Bangham, et al., (J. Mol. Biol. 13:238:252, 1965). A wide variety of phospholipids form MLV on hydration. MLV are composed of a number of bimolecular lamellae interspersed with an aqueous medium. The lipids or lipophilic substances are dissolved in an organic solvent. The solvent is removed under vacuum by rotary evaporation. The lipid residue forms a film on the wall of the container. An aqueous solution generally containing electrolytes and/or hydrophilic biologically active materials are added to the film. Agitation produces larger multilamellar vesicles. Small multilamellar vesicles can be prepared by sonication or sequential filtration through filters with decreasing pore size. Small unilamellar vesicles can be prepared by more extensive sonication. An improved method of encapsulating biologically active materials in multilamellar lipid vesicles is described in U.S. Pat. No. 4,485,054.
Unilamellar vesicles consist of a single spherical lipid bilayer entrapping aqueous solution. According to their size they are referred to as small unilamellar vesicles (SUV) with a diameter of 200 to 500 .ANG.; and large unilamellar vesicles (LUV) with a diameter of 1000 to 10,000.ANG.. The small lipid vesicles are restricted in terms of the aqueous space for encapsulation, and thus they have a very low encapsulation efficiency for water soluble biologically active components. The large unilamellar vesicles, on the other hand, encapsulate a high percentage of the initial aqueous phase and thus they can have a high encapsulation efficiency. Several techniques to make unilamellar vesicles have been reported. The sonication of an aqueous dispersion of phospholipid results in microvesicles (SUV) consisting of bilayer or phospholipid surrounding an aqueous space (Papahadjopoulos and Miller, Biochem. Biophys. Acta., 135: 624-238, 1968). In another technique (U.S. Pat. No. 4,089,801) a mixture of a lipid, an aqueous solution of the material to be encapsulated, and a liquid which is insoluble in water, is subjected to ultrasonication, whereby liposome precursors (aqueous globules enclosed in a monomolecular lipid layer), are formed. The lipid vesicles are then prepared by combining the first dispersion of liposome precursors with a second aqueous medium containing amphiphilic compounds, and then subjecting the mixture to centrifugation, whereby the globules are forced through the monomolecular lipid layer and forming the bimolecular lipid layer characteristic of liposomes.
Alternate methods for the preparation of small unilamellar vesicles that avoid the need of sonication are the ethanol injection technique (S. Batzri and E. D. Korn, Biochem. Biophys. Acta. 298: 1015-1019, 1973) and the ether injection technique (D. Deamer and A. D. Bangham, Biochem. Biophys. Acta. 443: 629-634, 1976). In these processes, the organic solution of lipids is rapidly injected into a buffer solution where it spontaneously forms liposomes--of the unilamellar type. The injection method is simple, rapid and gentle. However, it results in a relatively dilute preparation of liposomes and it provides low encapsulation efficiency. Another technique for making unilamellar vesicles is the so called detergent removal method (H. G. Weder and O. Zumbuehl, in "Liposome Technology" ed. G. Gregoriadis, CRC Press Inc., Boca Raton, Florida, Vol. I, Ch. 7, pg 79-107, 1984). In this process the lipids and additives are solubilized with detergents by agitation or sonication yielding defined micelles. The detergents are then removed by dialysis.
Multilamellar vesicles can be reduced both in size and in number of lamellae by extrusion through a small orifice under pressure, e.g., in a French press. The French press (Y. Barenholz; S. Amselem and D. Lichtenberg, FEBS Lett. 99: 210-214, 1979), extrusion is done at pressures of 20,000 lbs/in at low temperature. This is a simple, reproducible, nondestruction technique with relatively high encapsulation efficiency, however it requires multilamellar liposomes as a starting point, that could be altered to oligo- or unilamellar vesicles. Large unilamellar lipid vesicles (LUV) can be prepared by the reverse phase evaporation technique (U.S. Pat. No. 4,235,871, Papahadjopoulos). This technique consists of forming a water-in-oil emulsion of (a) the lipids in an organic solvent and (b) the substances to be encapsulated in an aqueous buffer solution. Removal of the organic solvent under reduced pressure produces a mixture which can then be converted to the lipid vesicles by agitation or by dispersion in an aqueous media.
U.S. Pat. No. 4,016,100, Suzuki, et al., describes still another method of entrapping certain biologically active materials in unilamellar lipid vesicles by freezing an aqueous phospholipid dispersion of the biologically active materials and lipids. All the above liposomes, made prior to 1983, can be classified either as multilamellar or unilamallar lipid vesicles. A newer type of liposomes is referred to as multivesicular liposomes (S. Kim, M. S. Turker, E. Y. Chi, S. Sela and G. M. Martin, Biochim. Biophys. Acta 728; 339-348, 1983). The multivesicular liposomes are spherical in shape and contain internal granular structures. A lipid bilayer forms the outermost membrane and the internal space is divided up into small compartments by bilayer septrum. This type of liposomes required the following composition: an amphiphatic lipid with net neutral charge, one with negative charge, cholesterol and a triacylglycerol. The aqueous phase containing the material to be encapsulated is added to the lipid phase which is dissolved in chloroform and diethyl ether, and a lipid-in-water emulsion is prepared as the first step in preparing multivesicular liposomes. Then a sucrose solution is shaken with the water-in-lipid emulsion; when the organic solvents are evaporated liposomes with multiple compartments are formed.
For a comprehensive review of types of liposome and methods for preparing them refer to a recent publication "Liposome Technology" Ed. by G. Gregoriadis. CRC Press Inc., Boca Raton, Florida, Vol. I, II, & III 1984.
Solutions are one of the oldest type of pharmaceutical dosage forms or drug delivery systems. A true solution is defined as a mixture of two or more components that form a homogeneous molecular dispersion, i.e., a one phase system. According to the United States Pharmacopeia, Twentieth Revision (USP XX, page 1027), solutions are liquid preparations that contain one or more soluble chemical substances usually dissolved in water. Further, solutions are used for the specific therapeutic effect of the solute, either internally or externally. Id.
Suspensions are preparations of finely divided, undissolved drugs dispersed in liquid vehicles (USP XX page 1030). In this sense a suspension is a heterogenous, two-phase system. Suspensions have been used as drug delivery systems for centuries, for providing an insoluble bioactive ingredient for oral, parenteral and for the topical route of administration. In the present multicomponent, multiphase liposomal system the biologically active substance is present in the solid form dispersed in the aqueous medium both inside and outside the lipid vesicles.
Hydrogels can be any one of a wide variety of synthetic and natural hydrophilic polymers. They are used in pharmaceutical dosage formulation for various purposes, i.e., as viscosity inducing agents for suspensions and ophthalmic solutions; as protective colloids to stabilize emulsions and suspensions; as vehicles for topically applied dosage forms; as controlled-release drug delivery systems (J. D. Andrade (ed) "Hydrogels" for Medical and Related Applications" ACS Symposium, Series Nol. 31 ACS Washington, D.C., 1976). A gel is generally a semisolid system of at least two constituents, consisting of a condensed mass enclosing and interpenetrated by a liquid. The gel mass may consist of flocculles of small particles or macromolecules existing as twisted, intermingled, matted strands. The polymer units are often bound together by electrostatic, hydrogen and van der Waal forces. Gels containing water are called hydrogels, those containing organic liquid are called organogels.
The hydrophilic polymers in aqueous media exhibit "pseudoplastic flow" due to the effect of intermolecular entanglements and the binding of water molecules. When the long randomly coiled polymer chain moves, their solvation layer (water of hydration) are dragged along which increases the resistance to flow, or the viscosity of the solution. This property is often utilized in pharmaceutical formulation to increase the viscosity of the preparation. Recently the hydrogels have been disclosed as useful for a controlled drug delivery system (S. W. Kim, Pharmacy International 4: 90-91 (1983)).